Micron scale tin oxide-based semiconductor devices

ABSTRACT

Micron scale tin oxide-based semiconductor devices are provided. Reactive-ion etching is used to produce a micron-scale electronic device using semiconductor films with tin oxides, such as barium stannate (BaSnO3). The electronic devices produced with this approach have high mobility, drain current, and on-off ratio without adversely affecting qualities of the tin oxide semiconductor, such as resistivity, electron or hole mobility, and surface roughness. In this manner, electronic devices, such as field-effect transistors (e.g., thin-film transistors (TFTs)), are produced having micron scale channel lengths and exhibiting complete depletion at room temperature.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 62/776,913, filed Dec. 7, 2018, the disclosure of which ishereby incorporated herein by reference in its entirety.

This application is related to concurrently filed U.S. patentapplication Ser. No. ______, filed on ______, entitled “PATTERNINGELECTRONIC DEVICES USING REACTIVE-ION ETCHING OF TIN OXIDES,” which isincorporated herein by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with government funds under Agreement No.HR0011-18-3-0004 awarded by The Defense Advanced Research ProjectsAgency (DARPA). The U.S. Government has certain rights in thisinvention.

FIELD OF THE DISCLOSURE

The present disclosure relates to semiconductor devices, such asfield-effect transistor devices, using tin oxide materials.

BACKGROUND

Transparent oxide thin-film transistors (TFTs) have been studiedextensively for over a decade for applications that include displays,wearable electronics, and smart windows. Due to the relatively highmobilities of the transparent oxide semiconductors used, when scaled tosmall dimensions transparent oxide TFTs are expected to have high draincurrent (I_(D), drain-to-source current per channel width) and low powerconsumption, which are important for energy-sensitive applications.Tin-based oxide systems are promising materials for transparent oxideTFTs due to the superior oxygen stability and high mobility of thesematerials at room temperature.

Among stannates (i.e., tin-based compounds), the transparent perovskitebarium stannate (BaSnO₃) is attracting worldwide attention following arecent discovery that lanthanum (La)-doped barium stannate has both highmobility at room temperature and excellent oxygen stability. Mobilitiesas high as 320 square centimeters per volt-second (cm²V⁻¹ s⁻¹) and 183cm²V⁻¹ s⁻¹ have been achieved in lanthanum-doped barium stannate singlecrystals and epitaxial thin films, respectively. The high mobility ofbarium stannate is attributed to its having a small effective massoriginating from a large dispersion of the conduction band from the 5sorbital of tin, a high dielectric constant (K≈20) which reduces dopantscattering, and small phonon scattering.

The performance of TFTs based on tin oxides has been steadily improvingin recent years, with the highest I_(D) using barium stannate achievedbeing 0.021 milliamps per micron (mA/μm). This performance is limited bythe large size of traditional tin oxide-based TFTs, which have channeldimensions of 100-200 microns (μm). These large devices have beenpatterned using metal shadow masks during growth, i.e., withoutphotolithography. Attempts to use photolithography to define smallerdevices have been plagued by the creation of oxygen vacancies in the tinoxide film, including in an undoped tin oxide buffer layer, during anion milling process. These vacancies make the undoped tin oxide bufferlayer conductive, shunt the device, and degrade performance.

SUMMARY

Micron (μm) scale tin oxide-based semiconductor devices are provided.Reactive-ion etching is used to produce a micron-scale electronic deviceusing semiconductor films with tin oxides, such as barium stannate(BaSnO₃). The electronic devices produced with this approach have highmobility, drain current, and on-off ratio without adversely affectingqualities of the tin oxide semiconductor, such as resistivity, electronor hole mobility, and surface roughness. In this manner, electronicdevices, such as field-effect transistors (e.g., thin film transistors(TFTs)), are produced having micron scale channel lengths and exhibitingcomplete depletion at room temperature.

An exemplary embodiment provides a semiconductor device. Thesemiconductor device includes a substrate and a semiconductor filmcomprising a tin oxide-based active semiconductor layer disposed overthe substrate. The semiconductor film is etched to form a semiconductorchannel having a channel length of less than 100 μm while preserving anelectrical property of the semiconductor film.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1A is an isometric cross-sectional view of an exemplarysemiconductor device having an active semiconductor layer disposed overa substrate according to embodiments disclosed herein.

FIG. 1B is a cross-sectional view of an exemplary reactive-ion etchingsystem for patterning the active semiconductor layer of thesemiconductor device of FIG. 1A.

FIG. 2A is a graphical representation of an etch rate of a tin oxidesemiconductor film and a ratio of resistance before and after etching,as a function of power applied to the reactive-ion etching system ofFIG. 1B.

FIG. 2B is a graphical representation of an etch rate of the tinoxide-based semiconductor film of FIG. 2A as a function of gas flow ratebetween chlorine (Cl₂) and argon (Ar) gases.

FIG. 3A is a cross-sectional view of another exemplary semiconductordevice having a tin oxide semiconductor film over a substrate.

FIG. 3B is a scanning electron microscope image of an embodiment of thesemiconductor device of FIG. 3A formed as a thin-film transistor (TFT)with a barium stannate thin film over a magnesium oxide substrate.

FIG. 4A is a graphical representation of a transfer characteristic ofthe semiconductor device of FIGS. 3A and 3B.

FIG. 4B is a graphical representation of an output characteristic of thesemiconductor device of FIGS. 3A and 3B.

FIG. 5 is a graphical representation of drain current dependence onchannel length for embodiments of the semiconductor device of FIGS. 3Aand 3B.

FIG. 6 is a graphical representation comparing drain current versuson-off ratio of embodiments of the semiconductor device of FIGS. 3A and3B among transparent oxide channel TFTs.

FIG. 7 is a flow diagram illustrating a process for patterning anelectronic device.

FIG. 8 is a schematic diagram of a generalized representation of anexemplary computer system that could be used to perform any of themethods or functions described above, such as patterning an activesemiconductor layer.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

Patterning electronic devices using reactive-ion etching of tin oxidesis provided. Reactive-ion etching facilitates patterning of tin oxides,such as barium stannate (BaSnO₃), at a consistent and controllable etchrate. The reactive-ion etching approach described herein facilitatesphotolithographic patterning of tin oxide-based semiconductors toproduce electronic devices, such as thin-film transistors (TFTs). Thisapproach further patterns a tin oxide-based semiconductor withoutadversely affecting its electrical properties (e.g., resistivity,electron or hole mobility), as well as maintaining surface roughness.This approach can be used to produce optically transparent devices withhigh drain current (I_(D), drain-to-source current per channel width)and high on-off ratio.

In addition, micron (μm) scale tin oxide-based semiconductor devices areprovided. Reactive-ion etching is used to produce a micron-scaleelectronic device using semiconductor films with tin oxides, such asbarium stannate. The electronic devices produced with this approach havehigh mobility, drain current, and on-off ratio without adverselyaffecting qualities of the tin oxide semiconductor, such as resistivity,electron or hole mobility, and surface roughness. In this manner,electronic devices, such as field-effect transistors (e.g., TFTs), areproduced having micron scale channel lengths and exhibiting completedepletion at room temperature.

FIG. 1A is an isometric cross-sectional view of an exemplarysemiconductor device 10 having an active semiconductor layer 12 disposedover a substrate 14 according to embodiments disclosed herein. Thesemiconductor device 10 can be a FET (e.g., an n-doped FET (NFET) or ap-doped FET (PFET)) having a drain 16, a source 18, and a channel 20between the drain 16 and the source 18, which is controlled by a gate22.

The active semiconductor layer 12 is grown or otherwise deposited on thesubstrate 14. A dielectric layer 24 is deposited over the activesemiconductor layer 12, and the gate 22 is formed over the dielectriclayer 24. Applying a potential to the gate 22 causes an electric fieldin the channel 20 to enable or disable current flow between the drain 16and the source 18. The active semiconductor layer 12 must be patternedto form the drain 16, source 18, channel 20, and other features of thesemiconductor device 10.

In an exemplary aspect, the active semiconductor layer 12 is formed withtin oxide-based semiconductors, such as tin oxide (SnO_(x)), bariumstannate, and strontium stannate (SrSnO₃). Such materials promise highelectron or hole mobility and low power consumption. In some examples,tin oxides can be used to produce optically transparent semiconductordevices 10. However, traditional approaches to patterning tin oxideshave been limited to channel dimensions of 100-200 μm due to use ofmetal shadow masks (rather than photolithography) during growth.Previous attempts to use photolithography using ion milling for etchingthe active semiconductor layer 12 to define smaller devices haveresulted in creation of oxygen vacancies in the semiconductor device 10,degrading performance.

In this regard, embodiments disclosed herein pattern the activesemiconductor layer 12 using reactive-ion etching to form thesemiconductor device 10. This facilitates photolithographic patterningof tin oxide-based semiconductors. Patterning the semiconductor device10 according to this approach does not adversely affect electricalproperties (e.g., resistivity, electron or hole mobility) of the activesemiconductor layer 12, and maintains surface roughness.

The semiconductor device 10 is illustrated as a top-gate FET. It shouldbe understood that this is for illustrative purposes, and embodimentsdisclosed herein can be arranged differently, such as a bottom-gate FET(e.g., having the gate 22 and dielectric layer 24 disposed between thesubstrate 14 and the active semiconductor layer 12). In addition,embodiments can provide other semiconductor devices 10, such as diodes,other transistor types (e.g., bipolar junction transistors), capacitors,inductors, resistors, and so on which are patterned using thereactive-ion etching disclosed herein.

FIG. 1B is a cross-sectional view of an exemplary reactive-ion etchingsystem 26 for patterning the active semiconductor layer 12 of thesemiconductor device 10 of FIG. 1A. The reactive-ion etching system 26includes a vacuum chamber 28 into which a first gas GAS1 and a secondgas GAS2 are injected. An electrode 30 is energized by a current source32, which may be a radio frequency (RF) current source operating at13.56 megahertz (MHz) (or another frequency in the in a 10-15 MHz rangeas appropriate) while a wall 34 of the vacuum chamber 28 is grounded.The electrode 30 generates an electric field in the vacuum chamber 28which ionizes the gas particles, creating a plasma. The plasma is usedto etch materials placed within the vacuum chamber 28.

In order to make micron scale TFTs, it is critical to be able tolithographically pattern the active semiconductor layer 12 withoutdegrading its electrical performance, particularly with tin oxides suchas barium stannate. A few methods have been used in the past to patternbarium stannate. Early barium stannate TFTs used shadow masks to definethe channel structure during deposition. This approach can patternbarium stannate without degrading its surface roughness and electricalproperties, but the channel length is limited to about 100 μm. Morerecently, ion milling has been employed to pattern barium stannate, butthe size of the channel length reported was about 100 μm. Importantly,the resulting TFT could not be depleted at room temperature and itson-off ratio was only 2. One reason for the inability to deplete the TFTat room temperature is that an undoped barium stannate buffer layerbelow the active semiconductor layer became conductive when subjected toion milling due to creation of oxygen vacancies created during the ionmilling process.

In order to overcome the limitations of previous techniques used todefine tin oxide-based TFTs, embodiments described herein use thereactive-ion etching system 26 to photolithographically patternsemiconductor devices 10, such as TFTs. In an exemplary aspect, a tinoxide semiconductor film 36, which includes the active semiconductorlayer 12 of FIG. 1A, is masked (e.g., by applying a photoresist layer)and placed on a platform 38 in the vacuum chamber 28, which may beconnected to or part of the electrode 30. As discussed further belowwith respect to FIGS. 2A and 2B, the reactive-ion etching system 26 isoperated to provide controllable etching of the tin oxide semiconductorfilm 36 by adjusting a power applied by the current source 32, a flowratio of the first gas GAS1 to the second gas GAS2, and pressure.Through this etching process, the tin oxide semiconductor film 36 may bepatterned to produce one or more semiconductor devices 10, such asdiodes, transistors (e.g., TFTs), capacitors, inductors, resistors, andso on. In this regard, embodiments of the reactive-ion etching system 26are controlled to preserve electrical properties (e.g., resistivity,mobility) and surface roughness (where preserving can mean no detectablechange or preserving within a 5% tolerance of values before etching).

One or more flow controls 40 (e.g., controllable valves) can providecontrol of the flow rate of each of the first gas GAS1 and the secondgas GAS2. In addition, one or more controllers 42 can control the flowcontrols 40 and/or control of the power applied by the current source32. In an exemplary aspect, the tin oxide semiconductor film 36 is abarium stannate film. However, the etching process can also be appliedto other tin oxides to achieve similar results.

The first gas GAS1 used in the reactive-ion etching system 26 is areactive gas, such as chlorine (Cl₂), which facilitates etching the tinoxide semiconductor film 36 with a chemical process rather than a purelyphysical process, in contrast to ion milling. Under appropriateconditions, the conductivity issue (e.g., in an undoped buffer layer)from the creation of oxygen vacancies can be resolved. Further, acontrollable etch rate is achieved that is not dependent on samplequality. This etching process for tin oxide semiconductor films 36provides a controllable etch rate while preserving surface roughness andelectrical resistivity and mobility—qualities critical to thefabrication of high-performance TFTs. In some examples, borontrichloride (BCl₃) or another gas can also be used for etching whilepreserving surface roughness and electrical properties, but the etchrate may be lower than when chlorine gas is used.

FIG. 2A is a graphical representation of an etch rate of the tin oxidesemiconductor film 36 and a ratio of resistance before and afteretching, as a function of power applied to the reactive-ion etchingsystem 26 of FIG. 1B. As illustrated, the etch rate increases when thepower applied by the current source 32 increases. Up to at least 100watts (W) of power, the etch rate increases without affecting theresistivity of the tin oxide semiconductor film 36.

However, the resistivity of the tin oxide semiconductor film 36 (e.g.,the resistivity of an undoped buffer layer, described further below withrespect to FIG. 3) starts to decrease when the power of the powerapplied by the current source 32 reaches 150 W. This is due to creationof oxygen vacancies when the tin oxide semiconductor film 36 isbombarded by a high energy plasma. Decreased resistivity reducesperformance of the semiconductor device 10 produced through the etchingprocess, such as by obstructing or preventing depletion of thesemiconductor device 10 at room temperature.

Accordingly, embodiments of the present disclosure control power appliedby the current source 32 to produce a desired etch rate whilemaintaining resistivity of the tin oxide semiconductor film 36. In thisregard, if the power is too low the etch rate may be undesirably lowsuch that etching to a desired depth requires additional time. Thispower can safely be increased up to 100 W, achieving an etch rate of 1.3nanometers per minute (nm/min) for barium stannate while maintaining theresistivity of the tin oxide semiconductor film 36.

FIG. 2B is a graphical representation of an etch rate of the tin oxidesemiconductor film 36 of FIG. 2A as a function of gas flow rate betweenchlorine (Cl₂) and argon (Ar) gases. As described above, the etch rateof the reactive-ion etching system 26 is also controlled by adjusting aflow ratio of the first gas to the second gas. In an exemplary aspect,the first gas is chlorine and the second gas is argon. In otherexamples, different gases may be used for etching, such as borontrichloride (BCl₃) in place of chlorine.

As illustrated, the etch rate is a function of a characteristicrelationship of the flow ratio of the first gas (chlorine) to the secondgas (argon). That is, the etch rate decreases as the flow ratio ofchlorine to argon increases, and the etch rate increases as the flowratio of chlorine to argon decreases. However, when 100% argon is used,the resistivity of the tin oxide semiconductor film 36 decreases. Thisdecreases performance of the semiconductor device 10 produced, asdescribed above with respect to FIG. 2A.

Accordingly, embodiments of the present disclosure select a desired etchrate and control the flow ratio of the first gas (chlorine or borontrichloride) to the second gas (argon) to produce the desired etch ratewhile maintaining resistivity of the tin oxide semiconductor film 36. Ata ratio of 3:1 of chlorine to argon, a tin oxide semiconductor film 36made with barium stannate may achieve its highest etch rate withoutdecreasing resistivity.

Embodiments of the present disclosure may achieve higher etch rates withhigher flow rates of argon and lower flow rates of chlorine or borontrichloride. However, using a higher ratio of argon does not preservethe resistivity reliably because of the high-energy ion bombardment ofargon ions. Thus using a higher ratio of chlorine is desired as itreliably preserves the resistivity of the etched tin oxide semiconductorfilm 36.

Without chlorine gas, the tin oxide semiconductor film 36 is exposed toargon gas directly and argon ion bombardment produces oxygen vacanciesin the tin oxide semiconductor film 36, decreasing resistivity. Thus,chlorine gas not only etches the tin oxide semiconductor film 36, butalso protects it from being exposed to argon gas directly by makingchlorine-based compounds. Lower overall gas pressure and high gas flowrate can facilitate ready evaporation of these compounds (e.g., bariumchloride and tin chloride). Accordingly, embodiments may use lowerpressures in the vacuum chamber 28, such as 1 milliTorr (mTorr). Theflow rate can be 15 standard cubic centimeters per minute (sccm) forchlorine gas and 5 sccm for argon gas, though other flow rates may beused.

FIG. 3A is a cross-sectional view of another exemplary semiconductordevice 10 having a tin oxide semiconductor film 36 over a substrate 14.The tin oxide semiconductor film 36 includes the active semiconductorlayer 12, which is reactive-ion etched as described with respect toFIGS. 1A-2B.

In this regard, embodiments provide the semiconductor device 10 with theactive semiconductor layer 12 etched to form the channel 20 having achannel length of less than 100 μm while preserving an electricalproperty of the tin oxide semiconductor film 36 (e.g., resistivity,electron or hole mobility). For example, by preserving resistivity ofthe tin oxide semiconductor film 36, a TFT formed according to thepresent disclosure can be completely isolated and depleted at roomtemperature.

In some examples, the tin oxide semiconductor film 36 includes a dopedactive semiconductor layer 12 over an undoped buffer layer 44 of thesame or a similar material. The tin oxide semiconductor film 36 can bedeposited over the substrate 14 using an appropriate technique, such asepitaxial growth. For example, the semiconductor device 10 can be a TFTformed with a barium stannate thin film (e.g., the tin oxidesemiconductor film 36) over a magnesium oxide (MgO) substrate (e.g., thesubstrate 14).

After the reactive-ion etching of the active semiconductor layer 12, asource electrode 46 and a drain electrode 48 can be deposited over theactive semiconductor layer 12. The source electrode 46 and drainelectrode 48 can be formed with an appropriate conductive material, suchas indium tin oxide (ITO, where optical transparency is desired) or ametal, such as copper (Cu), gold (Au), silver (Ag), aluminum (Al), etc.The source electrode 46 and drain electrode 48 can be deposited using anappropriate technique, such as sputtering, vapor deposition, plating,printing, etc.

In addition, the dielectric layer 24 for the gate can be deposited overthe active semiconductor layer 12 and/or the source electrode 46 anddrain electrode 48. The dielectric layer 24 can be an appropriatedielectric for performance of the TFT and/or transparency, such ashafnium oxide (HfO₂), barium hafnium oxide (BaHfO₃), lanthanum indiumoxide (LaInO₃), or other inorganic or organic dielectric materials. Thedielectric layer 24 can be deposited using an appropriate technique,such as atomic-layer deposition (ALD), spin on, spray on, vapordeposition, sputtering, etc. A gate electrode 50 is deposited over thedielectric layer 24, using an appropriate material (e.g., ITO) anddeposition technique (e.g., sputtering), such as used for the sourceelectrode 46 and drain electrode 48.

FIG. 3B is a scanning electron microscope image of an embodiment of thesemiconductor device 10 of FIG. 3A formed as a TFT using a bariumstannate thin film over a magnesium oxide substrate. The mobility andcarrier concentration of the as-grown active semiconductor layer 12 areμ=65.2 cm²V⁻¹ s⁻¹ and n=1.81×10¹⁹ cm⁻³, respectively, from Hallmeasurements at room temperature. The channel length and width of theTFT are L=0.3 μm and W=0.93 μm, respectively. This is the smallestbarium stannate-based TFT yet reported. Further scaling is limited bythe resolution of the light exposing tool (e.g., an Autostep 200 stepperwas used for the embodiment of FIG. 3A). As such, smaller devices may beachieved according to the etching methods described herein (e.g., byusing a higher resolution light exposing tool).

With reference to FIGS. 3A and 3B, in an exemplary embodiment a bariumstannate thin film (e.g., the tin oxide semiconductor film 36) is dopedwith lanthanum (La) (or another appropriate dopant). The barium stannatethin film is epitaxially grown on a magnesium oxide substrate (e.g., thesubstrate 14) by adsorption-controlled growth using molecular-beamepitaxy with barium, lanthanum, and tin oxide sources. In furtherdetail, an undoped barium stannate buffer layer 44 is grown over thesubstrate 14, followed by a lanthanum-doped barium stannate activesemiconductor layer 12. The channel 20 is formed using the activesemiconductor layer 12. The purpose of the buffer layer 44 is to reducethe density of threading dislocations that propagate into the overlyingdoped (and conducting) active semiconductor layer 12. Even with such abuffer layer 44, the threading dislocation density of barium stannatefilms grown on magnesium oxide as well as other commonly used substratesis of order 10¹¹ per centimeter (cm⁻²); an ideal substrate for bariumstannate has yet to be demonstrated, but several lattice matchedcandidates are being developed, such as Ba₂ScNbO₆ and (La,Nd)(Lu,Sc)O₃.It should be understood that the teachings of the present disclosureextend to such substrates as well.

In an exemplary aspect, the heterostructure of the tin oxidesemiconductor film 36 for TFT fabrication using barium stannate includesa 100-150 nanometer (nm) thick unintentionally doped buffer layer 44followed by approximately 10 nm (e.g., +/−5%) of doped activesemiconductor layer 12. In test structures, the Hall mobility of a10-nm-thick lanthanum-doped barium stannate active semiconductor layer12 (grown on a magnesium oxide substrate with an undoped barium stannatebuffer layer) reaches 184 square centimeters per volt-second (cm²V⁻¹s⁻¹) at a carrier concentration of 6.5×10¹⁹ per cubic centimeter (cm⁻³).Despite the thin tin oxide semiconductor film 36 and the use of amagnesium oxide substrate 14, this value is the highest mobilityreported for any barium stannate thin film using other substrates, suchas a dysprosium scandium oxide (DyScO₃) substrate.

The Hall mobility of a 10 nm thick lanthanum-doped barium stannateactive semiconductor layer 12 (on a 150 nm thick undoped barium stannatebuffer layer 44) with a carrier concentration of 1.3×10¹⁹ cm⁻³ is over90 cm²V⁻¹ s⁻¹. This decrease in mobility as the carrier concentration inthe active semiconductor layer 12 is reduced to below about 7×10¹⁹ cm⁻³is typical of barium stannate films and is a result of scattering fromthe huge density of threading dislocations, which become lesseffectively screened from the mobile charge carriers as the carrierconcentration is reduced. The mobility, limited by the threadingdislocations, is given by:

$\mu_{td} = {\frac{4\; {ec}^{2}}{Z^{2}\hslash \; N_{td}}( \frac{3n}{\pi^{4}} )^{2/3}\; ( {1 + {y(n)}} )^{3/2}}$where${y(n)} = {\frac{2\pi^{2}{\hslash ɛ}}{e^{2}m^{*}}( {3\pi^{2}n} )^{1/3}}$

and e, c, Z, N_(td), n, ε, and m* are the electron charge, c-axislattice parameter, charge state of a unit cell in a threadingdislocation, density of threading dislocations, carrier concentration,dielectric permittivity, and effective mass, respectively. Themobilities of this exemplary tin oxide semiconductor film 36 (e.g., withbarium stannate on magnesium oxide) are the highest yet reported at therelatively low carrier concentrations and low thickness of the activesemiconductor layer 12, which are needed to be able to fully deplete thechannel 20 of the semiconductor device 10 when a voltage is applied tothe gate 22.

The tin oxide semiconductor film 36 (e.g., barium stannate thin film) isreactive-ion etched to pattern one or more TFTs. The etching of the tinoxide semiconductor film 36 is achieved using the controllable etchingprocess described above, which preserves surface roughness andelectrical properties including resistivity and mobility. For deviceisolation, the tin oxide semiconductor film 36 was etched down about 18nm, considerably beyond the 10 nm thickness of the active semiconductorlayer 12 and well into the buffer layer 44. The thickness of thedielectric layer 24 was 20 nm.

The resulting micron-scale, photolithography defined tin oxide-based(e.g., barium stannate-based) transparent TFT has a peak field-effectmobility of 17.2 cm²V⁻¹ s⁻¹, an on-off ratio over 1.5×10⁸, drain currentI_(D) over 0.468 milliamps per micron (mA/μm) and a peaktransconductance (g_(m)) of 28.5 millisiemens per millimeter (mS/mm). Asdescribed further below, this is one of the highest performance fullytransparent oxide TFTs ever reported.

The large difference between the field-effect mobility and the Hallmobility can be attributed to the contact resistance. The contactresistance of ITO used in the source electrode 46 and the drainelectrode 48 of the embodiment of FIG. 3A is 0.51 ohm-centimeters (0cm). The normalized resistance of the active semiconductor layer 12 ofLa—BaSnO₃ with respect to the channel width is calculated by R_(s)×L,where R_(s) and L are sheet resistance and channel length, respectively.A sheet resistance of 5.4 kilohms per square (kΩ/sq) was measured by aHall measurement and the channel length is 0.3 μm, giving a channelresistance of 0.16 Ω·cm. When a drain-to-source voltage V_(DS)=1 volt(V) is applied, the actual voltage applied to the channel is given by

$V_{channel} = {{\frac{R_{channel}}{R_{ITO} + R_{channel}}\; V_{DS}} = {0.24\mspace{14mu} {V.}}}$

This is only a fraction of V_(DS) due to the high contact resistance ofthe ITO. Replacing V_(DS) by V_(channel) in the equation

$\mu_{FE} = {( \frac{L}{C_{ox}{WV}_{DS}} )\frac{\partial I_{DS}}{\partial V_{GS}}}$

yields the field-effect mobility, 72 cm²V⁻¹ s⁻¹. This is comparable tothe mobility of the as-grown channel layer (65.2 cm²V⁻¹ s⁻¹), which wasdetermined by the Hall effect. From the details of this calculation itis clear that a lower resistance contact would make V_(channel) closerto V_(DS) and increase the field-effect mobility and thus the draincurrent.

FIG. 4A is a graphical representation of a transfer characteristic ofthe semiconductor device 10 of FIGS. 3A and 3B. The drain current I_(D)and the field-effect mobility μ_(FE) are illustrated as a function ofgate-to-source voltage V_(GS). It can be seen that the bariumstannate-based TFT described above can be depleted completely at roomtemperature—a first for a photolithography defined tin oxide-based TFTor barium stannate TFT. The mobility of the device is calculated fromthe relation

${\mu_{FE} = {( \frac{L}{C_{ox}{WV}_{DS}} ){\frac{\partial I_{DS}}{\partial V_{GS}} \cdot I_{DS}}}},L,C_{ox},$

and W are drain-to-source current, the channel length, capacitance ofthe gate dielectric per unit area, and the channel width, respectively.

The capacitance of the gate dielectric (e.g., made with hafnium oxide)was measured with a metal-oxide-semiconductor (MOS) structure withdimensions of 50 μm×50 μm. The top and bottom electrode of the MOS wereITO and lanthanum-doped barium stannate. The capacitance decreasedslightly as frequency increases from 30 kilohertz (kHz) to 100 kHz. Forcalculating μ_(FE), the maximum Cox at the given voltage range has beenused so as to not overestimate μ_(FE). The calculated μ_(FE) is 17.2cm²V⁻¹ s⁻¹. There is dispersion in the capacitance at measurementfrequencies higher than 100 kHz and it is believed to be related to thetrapping of charge by defects or impurities within the hafnium oxidefilm (e.g., the dielectric layer 24) and at the interface between thedielectric layer 24 and the active semiconductor layer 12 since thecutoff frequency is 51.6 MHz. The cutoff frequency is calculated fromthe relation

$f = \frac{1}{2\pi \; {RC}}$

and is much higher than 100 kHz. The on-off ratio is over 1.5×10⁸. Thisis the highest on-off ratio among barium stannate TFTs when an undopedbarium stannate buffer layer 44 is used. The subthreshold swing has beencalculated from the relation

$S = ( \frac{{\partial\log}\; I_{DS}}{\partial V_{GS}} )^{- 1}$

and S is 0.15 volts per decade (V dec⁻¹).

The transconductance of the semiconductor device 10 (e.g., the TFT) at adrain-to-source voltage V_(DS)=1 V as a function of the gate-to-sourcevoltage V_(GS). The transconductance is calculated from the relation

$g_{m} = {\frac{\partial I_{DS}}{\partial V_{GS}}.}$

The maximum transconductance is 28.5 mS/mm at a drain-to-source voltageV_(DS)=1 V. This is much higher than the previous recordtransconductance in a barium stannate-based TFT, which is only 2 mS/mm.

FIG. 4B is a graphical representation of an output characteristic of thesemiconductor device 10 of FIGS. 3A and 3B. The output characteristic isillustrated with drain current I_(D) as a function of drain-to-sourcevoltage V_(DS) with varying gate-to source voltages V_(GS) from 2 V to−7 V. The drain current I_(D) reaches over 0.468 mA/μm. The slowincrease of the drain current I_(D) with respect to the drain-to-sourcevoltage V_(DS) is attributed to the high contact resistance of the ITOused in the source electrode 46 and the drain electrode 48. The highcontact resistance affects the slow response of the drain current I_(D)vs. the drain-to-source voltage V_(DS) in the measured outputcharacteristics; thus, the drain current I_(D) can be improved bylowering the contact resistance.

FIG. 5 is a graphical representation of drain current dependence onchannel length L_(CH) for embodiments of the semiconductor device 10 ofFIGS. 3A and 3B. The drain current I_(D) is inversely proportional tothe overall length L_(CH) of the channel 20 except at the shortestchannel length of 0.3 μm, showing little degradation with respect todevice scaling. The deviation from linear behavior at the short channellength of 0.3 μm in the inset of FIG. 5 is likely due to contactresistance rather than short channel effects (e.g., velocitysaturation).

The interface trap charge density D_(it) can be calculated using:

$S = {\frac{{kT}\; \ln \; 10}{e}( {1 + {\frac{e^{2}}{C_{ox}}D_{it}}} )}$

where k, T and e are the Boltzmann constant, temperature, and electroncharge, respectively. The calculated D_(it) of the TFT of FIGS. 3A and3B is 5.03×10¹² per electron volt-square centimeter (eV⁻¹ cm⁻²).

FIG. 6 is a graphical representation comparing drain current I_(D)versus on-off ratio (I_(on)/I_(off)) of embodiments of the semiconductordevice 10 of FIGS. 3A and 3B among transparent oxide channel TFTs. Inprior approaches, the high drain current I_(D) of the highestperformance TFT with a transparent oxide channel were achieved oncleaved and transferred flakes of beta gallium oxide (β-Ga₂O₃) onto anopaque substrate, which is not a scalable technology. These data areindicated as grey marks. Embodiments of the present disclosure with abarium stannate-based TFT exhibit among the best performances of alltransparent oxide TFTs, and is comparable to the best TFTs made withtransparent oxide channel materials (i.e., in device structures in whichother elements are not transparent). The drain current I_(D) of theseembodiments is second best among all fully transparent scalable oxidechannel TFTs.

FIG. 7 is a flow diagram illustrating a process for patterning anelectronic device. Dashed boxes represent optional steps. The processbegins at operation 700, with providing a substrate. The processcontinues at operation 702, with depositing an active semiconductorlayer comprising a tin oxide material over the substrate. In anexemplary aspect, the active semiconductor layer is deposited in asemiconductor film (e.g., a thin film). The semiconductor film mayinclude a doped active semiconductor layer and an undoped buffer layerbetween the active semiconductor layer and the substrate. In someexamples, the semiconductor film is formed with barium stannate.

The process continues at operation 704, with patterning the activesemiconductor layer. Operation 704 includes sub-operation 706, withmasking the active semiconductor layer. In an exemplary aspect, theactive semiconductor layer is masked via photolithography, such as byapplying a photoresist layer defining a semiconductor channel or otherfeatures of a transistor, diode, or other semiconductor device.Operation 704 continues at sub-operation 708, with reactive-ion etchingthe masked active semiconductor layer such that an electrical property(e.g., resistivity and/or mobility) of the active semiconductor layer ispreserved. In an exemplary aspect, the tin oxide-based activesemiconductor layer is etched at a controlled rate using a reactivefirst gas (e.g., chlorine) and a second gas (e.g., argon).

The process may optionally continue at operation 710, with depositing adielectric layer over the semiconductor channel. The process mayoptionally continue at operation 712, with depositing a sourceelectrode, a drain electrode, and a gate electrode over the etchedactive semiconductor layer. In an exemplary aspect, the source electrodeand the drain electrode are deposited before deposition of thedielectric layer. The gate electrode can be deposited over thedielectric layer and over the semiconductor channel.

Although the operations of FIG. 7 are illustrated in a series, this isfor illustrative purposes and the operations are not necessarily orderdependent. Some operations may be performed in a different order thanthat presented. For example, at least a portion of operation 712 (e.g.,deposition of the source electrode and the drain electrode) may beperformed before operation 710. Further, processes within the scope ofthis disclosure may include fewer or more steps than those illustratedin FIG. 7.

FIG. 8 is a schematic diagram of a generalized representation of anexemplary computer system 800 that could be used to perform any of themethods or functions described above, such as patterning an activesemiconductor layer. In some examples, the controller 42 of thereactive-ion etching system 26 of FIG. 1B is implemented as the computersystem 800. In this regard, the computer system 800 may be a circuit orcircuits included in an electronic board card, such as, a printedcircuit board (PCB), a server, a personal computer, a desktop computer,a laptop computer, an array of computers, a personal digital assistant(PDA), a computing pad, a mobile device, or any other device, and mayrepresent, for example, a server or a user's computer.

The exemplary computer system 800 in this embodiment includes aprocessing device 802 or processor, a main memory 804 (e.g., read-onlymemory (ROM), flash memory, dynamic random access memory (DRAM), such assynchronous DRAM (SDRAM), etc.), and a static memory 806 (e.g., flashmemory, static random access memory (SRAM), etc.), which may communicatewith each other via a data bus 808. Alternatively, the processing device802 may be connected to the main memory 804 and/or static memory 806directly or via some other connectivity means. In an exemplary aspect,the processing device 802 could be used to perform any of the methods orfunctions described above.

The processing device 802 represents one or more general-purposeprocessing devices, such as a microprocessor, central processing unit(CPU), or the like. More particularly, the processing device 802 may bea complex instruction set computing (CISC) microprocessor, a reducedinstruction set computing (RISC) microprocessor, a very long instructionword (VLIW) microprocessor, a processor implementing other instructionsets, or other processors implementing a combination of instructionsets. The processing device 802 is configured to execute processinglogic in instructions for performing the operations and steps discussedherein.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with the processing device 802, which may be amicroprocessor, field programmable gate array (FPGA), a digital signalprocessor (DSP), an application-specific integrated circuit (ASIC), orother programmable logic device, a discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. Furthermore, the processingdevice 802 may be a microprocessor, or may be any conventionalprocessor, controller, microcontroller, or state machine. The processingdevice 802 may also be implemented as a combination of computing devices(e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration).

The computer system 800 may further include a network interface device810. The computer system 800 also may or may not include an input 812,configured to receive input and selections to be communicated to thecomputer system 800 when executing instructions. The input 812 mayinclude, but not be limited to, a touch sensor (e.g., a touch display),an alphanumeric input device (e.g., a keyboard), and/or a cursor controldevice (e.g., a mouse). The computer system 800 also may or may notinclude an output 814, including but not limited to a display, a videodisplay unit (e.g., a liquid crystal display (LCD) or a cathode ray tube(CRT)), or a printer. In some examples, some or all inputs 812 andoutputs 814 may be combination input/output devices.

The computer system 800 may or may not include a data storage devicethat includes instructions 816 stored in a computer-readable medium 818.The instructions 816 may also reside, completely or at least partially,within the main memory 804 and/or within the processing device 802during execution thereof by the computer system 800, the main memory804, and the processing device 802 also constituting computer-readablemedium. The instructions 816 may further be transmitted or received viathe network interface device 810.

While the computer-readable medium 818 is shown in an exemplaryembodiment to be a single medium, the term “computer-readable medium”should be taken to include a single medium or multiple media (e.g., acentralized or distributed database, and/or associated caches andservers) that store the one or more sets of instructions 816. The term“computer-readable medium” shall also be taken to include any mediumthat is capable of storing, encoding, or carrying a set of instructionsfor execution by the processing device 802 and that causes theprocessing device 802 to perform any one or more of the methodologies ofthe embodiments disclosed herein. The term “computer-readable medium”shall accordingly be taken to include, but not be limited to,solid-state memories, optical medium, and magnetic medium.

The operational steps described in any of the exemplary embodimentsherein are described to provide examples and discussion. The operationsdescribed may be performed in numerous different sequences other thanthe illustrated sequences. Furthermore, operations described in a singleoperational step may actually be performed in a number of differentsteps. Additionally, one or more operational steps discussed in theexemplary embodiments may be combined.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;and a semiconductor film comprising a tin oxide-based activesemiconductor layer disposed over the substrate; wherein thesemiconductor film is etched to form a semiconductor channel having achannel length of less than 100 microns (μm) while preserving anelectrical property of the semiconductor film.
 2. The semiconductordevice of claim 1, wherein the preserved electrical property is aresistivity of the semiconductor film.
 3. The semiconductor device ofclaim 1, wherein the preserved electrical property is an electronmobility or a hole mobility of the semiconductor film.
 4. Thesemiconductor device of claim 1, wherein the electrical property of thesemiconductor film is preserved within a 5% tolerance of its valuebefore etching.
 5. The method of claim 1, wherein a surface roughness ofthe semiconductor film is preserved.
 6. The semiconductor device ofclaim 1, wherein the semiconductor film defines a thin-film transistor(TFT).
 7. The semiconductor device of claim 6, wherein the TFT can becompletely depleted at room temperature.
 8. The semiconductor device ofclaim 6, wherein the semiconductor film further comprises: an undopedtin oxide-based buffer layer disposed on the substrate; and the activesemiconductor layer disposed on the buffer layer.
 9. The semiconductordevice of claim 8, wherein a source, a drain, and a channel are definedin the active semiconductor layer.
 10. The semiconductor device of claim9, further comprising a source electrode disposed over the source and adrain electrode disposed over the drain.
 11. The semiconductor device ofclaim 10, further comprising: a dielectric layer disposed over thechannel; a gate electrode disposed over the dielectric layer.
 12. Thesemiconductor device of claim 11, wherein a channel length of thechannel is less than 1 μm.
 13. The semiconductor device of claim 11,wherein: the source electrode, the drain electrode, and the gateelectrode comprise indium tin oxide (ITO); and the dielectric layercomprises hafnium oxide (HfO₂).
 14. The semiconductor device of claim 1,wherein the semiconductor device is optically transparent.
 15. Thesemiconductor device of claim 1, wherein the semiconductor film furthercomprises: an undoped tin oxide-based buffer layer disposed on thesubstrate; and the active semiconductor layer disposed on the bufferlayer.
 16. The semiconductor device of claim 15, wherein the activesemiconductor layer and the buffer layer comprise barium stannate(BaSnO₃).
 17. The semiconductor device of claim 16, wherein the activesemiconductor layer is doped with lanthanum (La).
 18. The semiconductordevice of claim 17, wherein the active semiconductor layer isepitaxially grown over the buffer layer.
 19. The semiconductor device ofclaim 16, wherein the substrate comprises magnesium oxide (MgO).